U.S. Pat. No. 6,658,970 discloses a conventional screwdriver including a shaft-mounting seat, a handle, a ratchet assembly, a drive shaft, two pawls and a tubular sleeve. The shaft-mounting seat is connected fixedly to the handle. The ratchet assembly is connected to the drive shaft, and is disposed in the shaft-mounting seat. The pawls are mounted movably in the shaft-mounting seat, and are operable to engage removably the ratchet assembly. The tubular sleeve is mounted around the shaft-mounting seat, and is operable for switching states of the pawls such that the conventional screwdriver is convertible to serve as a ratcheting screwdriver or a non-ratcheting screwdriver. However, in use, the rotating speed of the drive shaft is the same as that of the handle, so that the conventional screwdriver may be laborious.
As shown in FIGS. 1 and 2, U.S. Pat. No. 5,406,866 discloses a conventional speed-selectable screwdriver 10 including a handle 11, an insert 12, a roller cage 13, a plurality of first rollers 14, a plurality of second rollers 15, a limiting spring 16, a knob 17, a drive shaft 18, an overdrive sleeve 19, two triangular mounting plates 20, three gear assemblies 21, a housing 22, a bit 23 and a bit holder 24. The insert 12 is mounted fixedly in the handle 11, and is formed with a non-circular hole 121. The roller cage 13 is mounted rotatably in the non-circular hole 121, and is formed with a plurality of channels 131. The limiting spring 16 is mounted to the roller cage 13 for limiting the relative movement between the roller cage 13 and the insert 12. The knob 17 is mounted fixedly around a front portion of the roller cage 13. The drive shaft 18 is inserted into the roller cage 13, and has a splined front end 181 and an enlarged body portion 182. The overdrive sleeve 19 is sleeved on the drive shaft 18, is located between the splined from end 181 and the enlarged body portion 182 of the drive shaft 18, and has a sun gear portion 191. The first rollers 14 are mounted respectively and movably in the channels 131 of the roller cage 13, and are contactable with the enlarged body portion 182 of the drive shaft 18. The second rollers 15 are mounted respectively and movably in the channels 131 of the roller cage 13, and are contactable with the overdrive sleeve 19. The mounting plates 20 are mounted rotatably and respectively on the drive shaft 18 and the overdrive sleeve 19. Each of the gear assemblies 21 is mounted rotatably between the mounting plates 20, and has a planetary gear 211 that meshes with the sun gear portion 191 of the overdrive sleeve 19, and an auxiliary gear 212 that is co-rotatable with the planetary gear 211 and that meshes with the splined front end 181 of the drive shaft 18. The housing 22 is mounted fixedly around the mounting plates 20. The bit holder 24 is connected co-rotatably to the drive shaft 18. The bit 23 is mounted co-rotatably to the bit holder 24.
Referring further to FIG. 3, the insert 12 has a plurality of cam surfaces 122 and a plurality of slip surfaces 123. The cam surfaces 122 and the slip surfaces 123 are arranged alternately in the circumferential direction, and cooperatively define the non-circular hole 121.
For each of the gear assemblies 21, the number of teeth of the auxiliary gear 212 is two times the number of teeth of the planetary gear 211, and is the same as that of the sun gear portion 191 of the overdrive sleeve 19. The number of teeth of the planetary gear 211 of each of the gear assemblies 21 is the same as that of the splined front end 181 of the drive shaft 18.
When a user holds firmly the housing 22 and turns the handle 11 relative to the housing 22 in a first rotational direction (D1, see FIG. 3), the cam surfaces 122 of the insert 12 respectively push the second rollers 15 to be in frictional contact with the overdrive sleeve 19, such that the overdrive sleeve 19 is co-rotatable with the handle 11. By virtue of the gear assemblies 21, the drive shaft 18 is rotated in the first rotational direction (D1) at a rotational speed four times the rotational speed of the overdrive sleeve 19. At this time, an outer surrounding surface of the enlarged body portion 182 of the drive shaft 18 pushes the first rollers 14 to correspond respectively in angular position to the slip surfaces 123 so as to permit the relative rotation between the drive shaft 18 and the insert 12.
When the user turns the handle 11 in the first rotational direction (D1) without holding firmly the housing 22, the cam surfaces 122 of the insert 12 respectively push the first rollers 14 to be in frictional contact with the enlarged body portion. 182 of the drive shaft 18, such that the drive shaft 18 is rotated by the handle 11 at the same rotational speed. Additionally, when the user turns the handle 11 in a second rotational direction (D2, see FIG. 3) without holding firmly the housing 22, the slip surfaces 123 are rotated to correspond respectively in angular position to first rollers 14, such that the drive shaft 18 would not be rotated by the handle 11.
However, the conventional speed-selectable screwdriver 10 cannot serve as a non-ratcheting screwdriver, and has a relatively complex structure. Moreover, since the handle 11 rotates the drive shaft 18 by virtue of the frictional contact between each of the first rollers 14 and the enlarged body portion 182 of the drive shaft 18, or the frictional contact between each of the second rollers 15 and the overdrive sleeve 19, an output torque of the drive shaft 18 may be insufficiently large.